A technique of forming an insulator for a spark plug by injection molding using a material obtained by mixing ceramic and resin has been known (e.g., German Patent Application Laid-Open Publication No. 102010042155 (DE10 2010 042 155 A1) and German Patent Application Laid-Open Publication No. 102012200045 (DE10 2012 200 045 A1)).
According to the technique disclosed in German Patent Application Laid-Open Publication No. 102010042155 (DE10 2010 042 155 A1), injection of a material into a cavity of a mold is performed from a position corresponding to a front end, in an axial direction, of an insulator. According to the technique disclosed in German Patent Application Laid-Open Publication No. 102012200045 (DE10 2012 200 045 A1), injection of a material into a cavity of a mold is performed from one position corresponding to a portion, having a maximum outer diameter, of an insulator.
In the above-described techniques, however, the material injection position is not sufficiently contrived, and there is a possibility of reduction in dielectric strength properties of manufactured insulators. For example, the material injection position of German Patent Application Laid-Open Publication No. 102010042155 (DE10 2010 042 155 A1) may cause insufficient density of a rear end portion, of the insulator, farthest from the injection position, and the insufficient density may reduce dielectric strength property of the rear end portion of the insulator. Meanwhile, in the German Patent Application Laid-Open Publication No. 102012200045 (DE10 2012 200 045 A1), since the material injection position is only one, the distance in which the material moves to reach a rear end portion or a front end portion of the insulator is long. As a result, density of the front end portion or the rear end portion of the insulator is insufficient, and the insufficient density may reduce dielectric strength property of the front end portion or the rear end portion of the insulator.
An advantage of the present invention is to suppress reduction in dielectric strength property of an insulator when the insulator is formed by injection molding.